Biological Soil Crust: Holding the Place in Place

Despite its rugged appearance, Red Rock Country is a sensitive and limited living system in need of your help to sustain and enhance it. In this fragile desert, plants grow slowly and are easily trampled, and soils wash away from the impact of thousands of human footprints. Many popular areas have become crowded and show signs of damage to vegetation, cryptobiotic soil, water, archaeological sites and wildlife habitat.

Although the soil surface may look like dirt to you, it is full of living organisms that are a vital part of desert ecosystems. The large, black patches of dark, knobby, brittle crust are actually a cryptobiotic crust. Communities of soil crust are made of lichens, mosses, cyanobacteria, liverwort and fungi. The crust prevents erosion and retains water and minerals for plants. Without this veneer of life within the soil crust, the red rock landscape which Sedona is famous for would become barren sand dunes.

Even a single footprint off-trail on fragile biological soil crust can have a long-lasting effect on the desert ecosystem. Because most living crust biomass is concentrated in the top 1/8th inch (3 mm) of the soil, even small impacts have profound consequences. Crushed crusts contribute less nitrogen and organic matter to the ecosystem and the soils are left highly susceptible to both wind and water erosion. Recovery may take up to 250 years in places of lower rainfall, assuming an area is not again disturbed.

These crusts are found all over the world and they generally cover all soil spaces not occupied by trees, shrubs or grasses. In many areas, they comprise over 70% of the living ground cover and are crucial in reducing erosion, increasing water retention, and increasing soil fertility. In most dry regions, these crusts are dominated by cyanobacteria (blue-green algae), which are one of the oldest known life forms. These living organisms and their by-products create a continuous crust on the soil surface that helps, well, hold the place in place. They are also an important pioneer stage in succession on bare ground, thus enabling grasses and herbs to become established.

Soil loss due to rainfall and water movement is increased when cyanobacteria connections are broken. This is particularly problematic when the impact is in a continuous strip, such as a vehicle or bicycle track, because channels for water flow are quickly formed, especially on slopes. Soil crusts are also important in the absorption of rainfall. This function is especially vital in arid areas that experience sporadic, heavy rainfall, like the Southwestern United States during the summer monsoon. When it rains, the organisms and their mucilage absorb up to ten times their volume in water and then release the water slowly into the soil once the rain ends.

Moreover, because plant cover is sparse, crusts are an important source of organic matter for desert soils. Organic matter is an important food source for organisms that live below the soil surface and who help keep nutrients available for plants by decomposing plant litter. Cyanobacteria and cyanolichens contribute nitrogen to soils, which is especially important in desert ecosystems where nitrogen often limits plant growth. Indirect benefits to soil health also are evident.

Many human activities are harmful to biological crusts. The crusts are no match for the compressional stress caused by footprints of livestock and people or by the tires of bicycles and vehicles. You may ask yourself, ‘What Can I Do to Preserve a Healthy Desert Ecosystem?’ When you are outdoors, whether you are hiking or biking, stay on established trails. Creating new trails destroys the fragile biological soil crust, desert vegetation and animal habitat. If you must travel off trail, choose slickrock, gravel or sand surfaces. At trailheads, do not go beyond piles of rock, logs or fences. These have been placed to prevent further damage. You may also consider volunteering for trail-maintenance crews and learn techniques for repairing unmaintained trails. It is only through our diligent efforts and respect for the land that we will be able to preserve this beautiful, ever-changing desert ecosystem for future generations to appreciate.

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